1. Field of the Invention
This invention relates to a process for producing gypsum boards and also to an apparatus for producing gypsum boards.
2. Discussion of the Background
Gypsum boards have a plate-like structure with a gypsum rock core covered on its upper and lower surfaces and preferably, its left and right side surfaces by sheets of lining paper for gypsum boards, and are extensively used as a building material. Such gypsum boards are produced by holding a gypsum slurry at upper and lower surfaces thereof with an overlay paper and an underlay paper, respectively, and then causing a resulting preform to travel between upper and lower forming units while pressing the preform with the upper forming unit. As the upper forming unit and lower forming unit, forming plates and plates are used. Using forming rolls and/or plates in combination, gypsum boards are produced as a through process, for example, by a gypsum board forming apparatus 15 shown in FIG. 5.
A description will specifically be made with reference to FIG. 5. To cover an upper surface and lower surface of a gypsum rock core, respectively, an overlay paper 1, the direction of which has been changed by a roller 4, and an underlay paper 2 are continuously fed from a right side as viewed in the drawing. A gypsum slurry 3, which has a predetermined viscosity, is fed onto the underlay paper 2, and is adjusted to a predetermined thickness by the roller 4. Between an upper plate 5 and a lower plate 6, a green gypsum board which has a predetermined thickness and is planer is formed under a pressure from the upper plate 5. The green gypsum board is then caused to travel leftwards by a belt conveyor (not shown) arranged on a left side as viewed in the drawing. After that, the green gypsum board is processed or treated through steps such as drying, a gypsum board is produced as a final product. Incidentally, in FIG. 5, a predetermined forming platen which serves to support the underlay paper 2 is generally arranged on a right side of the lower plate 6, and the underlay paper 2 is allowed to slide on the forming platen and to move leftwards.
When gypsum boards are produced by such process and apparatus, catching of an undesired object for a certain cause, such as a lump of gypsum in the gypsum slurry, between the upper plate 5 and the lower plate 6 causes a problem that, as the gypsum board advances, the overlay paper 1 or underlay paper 2 is caused to progressively tear due to the undesired object. If the overlay paper 1, the underlay paper 2 and the gypsum slurry 3 are continuously fed while leaving such a tear as is, the paper continues to tear so that the production of defective products continues. This eventually leads to a problem in that the paper is torn off and the continuous operation for the production of gypsum boards cannot be continued any longer.
If an undesired object is caught between the upper plate 5 and the lower plate 6 in during production of gypsum boards and the overlay paper 1 and/or underlay paper 2 tears, it is therefore necessary to immediately stop the producing operation, to remove the undesired object, to insert fresh papers or a fresh paper between the upper plate 5 and the lower plate 6, and then to feed the gypsum slurry 3 to resume the operation. As is readily appreciated from the foregoing, tearing of the overlay paper 1 and/or underlay paper 2 during production leads to a substantial reduction in the continuous productivity of gypsum boards. It has hence been desired to develop an efficient method for making it possible to promptly discover tearing of the overlay paper 1 and/or underlay paper 2 and further to cope with the tearing.
An object of the present invention is, therefore, to provide a process for producing gypsum boards, which, if an overlay paper and/or an underlay paper locally tears due to a mixed undesired object during continuous production of gypsum boards, promptly discovers the tearing, thereby preventing the localized tearing from developing into such a tear as extending over the entire width of the paper and hence, permitting efficient production of gypsum boards without stopping the production. Another object of the present invention is to provide an apparatus for producing gypsum boards, which can effectively practice this production process of gypsum boards.
To achieve the above-described objects, the present invention provides a process for producing gypsum boards by holding a gypsum slurry at upper and lower surfaces thereof with an overlay paper and an underlay paper, respectively, and then causing a resulting preform to travel between upper and lower forming units, which are composed in combination of plates and/or forming rolls as an upper forming unit and a lower forming unit, while pressing the preform with the upper forming unit, which comprises: forming an electric circuit at least one of between the upper forming unit and the lower forming unit, between a part of the overlay paper, said part being located in an advancing direction of the production of the gypsum boards as viewed from the upper forming unit, and the upper forming unit, and between a part of the underlay paper, said part being located in the advancing direction of the production of the gypsum boards as viewed from the lower forming unit, and the lower forming unit, applying a voltage across the electric circuit, widening a spacing between the upper forming unit and the lower forming unit when a current has flowed through the electric circuit during production of the gypsum boards, and subsequent to elimination of a cause for the flow of the current, narrowing the widened spacing back to the spacing before the widening.
The present invention also provides an apparatus for producing gypsum boards by holding a gypsum slurry at upper and lower surfaces thereof with an overlay paper and an underlay paper, respectively, and then causing a resulting preform to travel between upper and lower forming units, which are composed in combination of plates and/or forming rolls as an upper forming unit and a lower forming unit, while pressing the preform with the upper forming unit, which comprises: an electric circuit formed at least one of between the upper forming unit and the lower forming unit, between a part of the overlay paper, said part being located in an advancing direction of the production of the gypsum boards as viewed from the forming unit, and the upper forming unit, and between a part of the underlay paper, said part being located in the advancing direction of the production of the gypsum boards as viewed from the lower forming unit, and the lower forming unit; a current detector for detecting flowing of a current through the electric circuit and outputting a detection signal; and a device for widening, responsive to the detection signal, a spacing between the upper forming unit and the lower forming unit and after an elapse of a predetermined time, narrowing the widened spacing back to the spacing before the widening.